A drop-in replacement for Send-MailMessage. You can get a 100 email/day sendgrid API key for free at https://sendgrid.com

Send-SendGridMailMessage

function Send-SendGridMailMessage {
    [CmdletBinding()]
    param (
        [Parameter(Mandatory)][String[]]$To,
        [Parameter(Mandatory)][String]$Subject,
        [String]$Body,
        [Parameter(Mandatory)][String]$From,
        [Switch]$BodyAsHtml,
        [String]$SendGridApiKey = $env:SENDGRID_API_KEY
    )
    if (-not $SendGridApiKey) {
        throw "You need to supply a Sendgrid API key either via the -SendGridApiKey parameter or by setting the SENDGRID_API_KEY environment variable"
    }

    $SendGridMessage = @{
        personalizations = @()
        from = @{email=$from}
        content = @(@{
            type  = if ($BodyAsHtml) {'text/html'} else {'text/plain'}
            value = $Body
        })
    }

    $Personalization = @{
        to      = @()
        subject = $Subject
    }

    $To.foreach{
        $Personalization.To += @{email=$PSItem}
    }

    $SendGridMessage.personalizations += $Personalization

    $InvokeRestMethodParams = @{
        URI            = 'https://api.sendgrid.com/v3/mail/send'
        Authentication = 'Bearer'
        Token          = (Convertto-securestring -asplaintext -force $SendGridApiKey)
        Method         = 'POST'
        Body           = ($SendGridMessage | ConvertTo-Json -Depth 5)
        ContentType    = 'application/json'
    }

    Invoke-RestMethod @InvokeRestMethodParams
    write-verbose ($sendgridmessage | ConvertTo-Json -depth 5)
}